A dynamic programming language performs behaviors at run time that other (static programming) languages typically perform during compilation, if at all. Behaviors performed at runtime by dynamic languages include extending the program by adding new code, by creating objects and definitions, by modifying the type system during program execution and so on.
A dynamic programming language is typically dynamically typed, meaning that the type checking is performed at runtime. In dynamic typing, types are associated with values resulting from execution of the program. Dynamically typed languages include Clojure, Groovy, JavaScript, Lisp, Objective-C, PHP, Prolog, Python, Ruby, Smalltalk, late-binding in Visual Basic, IronPython and IronRuby. Dynamic typing is less rigid than static typing but can result in a greater potential for execution errors (e.g., an error results because a value for a variable has a type that is not allowed). Dynamically typed language systems typically make fewer compile time checks on the source code. Runtime checks potentially can be more sophisticated because they can use dynamic (runtime) information in addition to information that was present during compilation and is still available at runtime. Runtime checks assert that conditions hold in a particular execution of the program and are repeated for each execution of the program.
Binding selects which actual operation to use when a syntactic operation is applied in source code. Binding can occur either at compile time, in which case it is called “static binding”, or can occur dynamically at runtime, which is called “dynamic binding”. Dynamic binding postpones the resolving of undefined variables until a program is run. Dynamic binding is different from static binding because its result—the meaning assigned to an operation, for example—depends on the runtime types of the actual values it operates on instead of on the compile time types of the variables in the source code. Typically, statically typed programming languages perform static binding and dynamically typed programming languages perform dynamic binding. However, it is possible to have a hybrid of the two where a static language contains dynamic binding (such as C#4.0 with the dynamic language feature) and vice versa.
Test-driven development refers to an iterative software development technique in which a developer first writes a test case that validates a desired improvement or new function, then the developer produces the code that causes the test to pass, followed by refactoring the code base to account for any new system requirements. For example, code that includes the logic that manipulates objects may be written before the code that defines the objects is written. Another way of saying this is: consuming (consumption) code is written before the objects that are consumed are written.